It's the Muiredach Cross which dates from the 10th century. The cross stands in the middle of the ruins of the medieval monastery of Monasterboice, which was founded by a little known disciple of St. Patrick known as St. Buite. Anybody ever hear of St. Buite?
It is massive--17 feet 8 inches tall. One side of the cross depicts scenes from the Old Testament and one side stories from the New Testament. The monks used it to teach the Bible because the people couldn't read the Latin in which the Bible was written at the time and also couldn't read anything else.
The cross is in the middle of an ancient graveyard that you can see in the background. It is full of different styles of Celtic crosses.